Displacement check options (pump style?)

[gschuld]

I know some have used special gauges that measure bore diameter through the spark plug hole but only if the head is a center plug design and all plugs are straight shot accessible.  But for those that are not, is it OK to use a cubic inch tester common and trusted in the car world for many years?

Speedway, Comp Cams, and P&G are examples.

Of course the valvetrain would have to be disabled for it to work(keeping both valves closed for the length of stroke).  But beyond that, is there a logistical reason this would not be used for motorcycle engines?  I’ve been told that anyone can supply their own method for testing displacement as long as it can be proven to be accurate and repeatable.

George

[dw230]

The SCTA rule book details the use of the pump. The engine must be within 3% of the upper or lower class limit. This limit is imposed because of the accuracy of the pump unit.

I do not know how BMST applies the pump when doing displacement checks.

DW 

[55chevr]

At BMST impound we have always torn down to have displacement verified with a micrometer. 

JD

[**RP**]

Ten years ago AMA allowed the retracting gauge thru a spark plug hole method ……. not sure what the current rules interpretation allow.  Always best to contact the tech official for an answer.

[Stainless1]

The SCTA had also used an oil pump, way more accurate for measuring the smaller motorcycle cylinders than the air pump... But you end up with a cylinder with a lot of ATF that makes the motor a little hard to start and smoking like you have a mosquito fogger.
Larger Motorcycles, are measured by air... talking ones with 40ci per cylinder... pump accuracy
The AMA and FIM both require direct measurement if I remember correctly.
Yes Drew measured bore and stroke with the scissor tool, for center plug motors, does not do well with offset plugs.

[Koncretekid]

At BMST impound we have always torn down to have displacement verified with a micrometer. 

JD

I didn't see any micrometers when they measured my BSA - - digital calipers instead!

Dennis should be able to tell you how they measured his CB750.  Be prepared to pull the head.

Tom

[gschuld]

I know what Dennis and others had to do.  I’m just curious if there is another option possible by going through the effort of bringing special equipment for the job.  The air pump style displacement checkers are well respected and widely used tools for measuring displacement.  Perhaps a custom one dedicated for the smaller cylinder volumes of multi cylinder motorcycle engines could be assembled and be proven accurate enough to be used. 

George